1. Field of the Invention
The present invention relates to an air vent structure for a plug cap applied to a DOHC (double over head camshaft) type gasoline engine and the like.
2. Description of the Prior Art
This type of air vent structure is disclosed in, for example, Japanese Utility Model Application Laid-Open No. 5-59786 (1993). As shown in FIGS. 12 and 13, a plug cap 1 comprises a plug cap body 2 and a rain cover 3. The plug cap 1 is inserted into a spark plug mounting hole 4 to connect an ignition cable 7 to a spark plug 6 mounted to an engine 5 and to close an opening end of the spark plug mounting hole 4 by the rain cover 3.
A vent channel 8 is formed between an outer peripheral surface of an upper end portion of the plug cap body 2 and an inner peripheral surface of the rain cover 3, and an air vent aperture 9 is formed at an upper end of the rain cover 3 for communication between the vent channel 8 and the atmosphere.
A spherical sealing member storage chamber 10 as a larger space is formed in an intermediate portion of the air vent aperture 9, and a sealing sphere 11 is received in the sealing member storage chamber 10.
The sealing sphere 11 normally shuts off the communication through the air vent aperture 9 by gravity, and prevents water from entering the spark plug mounting hole 4 when water enters the sealing member storage chamber 10 from above by a high-pressure jet of water.
When the temperature of the engine 5 rises and air in the spark plug mounting hole 4 thermally expands to increase the internal pressure, the expanded air pushes the movable sealing sphere 11 upwardly to escape outwardly. This prevents an increase in internal pressure of the spark plug mounting hole 4.
However, the conventional structure is constructed such that water enters up to the sealing member storage chamber 10. When the spark plug mounting hole 4 is disposed at a slant, the air vent aperture 9 is slanted as shown in FIG. 14 and the air vent aperture 9 is not shut off by gravity exerted upon the sealing sphere 11, resulting in water in the sealing member storage chamber 10 entering the spark plug mounting hole 4.